Example: 150 mm fall over 10 m
The ratio is 0.150 ÷ 10 = 0.015. Multiply by 100 for 1.5%; invert for a vertical:horizontal ratio of 1:66.6667; arctan(0.015) is about 0.859°. Conversely, a design requirement of 1.5% over a 4 m segment corresponds to 4 × 0.015 = 0.06 m, or 60 mm of vertical change.
Convert the rise and run into the same unit
If the level difference is measured in millimetres and the plan run in metres, convert one before dividing. A 150 mm change is 0.150 m. Dividing 150 by 10 without that conversion would overstate the ratio by a factor of 1,000.
The run is the horizontal distance, not the length measured along the sloping surface. On a gentle slope the difference may look small, but using consistent geometry avoids compounding errors when comparing steeper ramps or roof surfaces.
Percent, ratio and angle
A 1.5% slope rises or falls 1.5 units vertically per 100 equal units horizontally. In the convention 1:n, the first number is vertical and n is horizontal, so n = run ÷ rise. Label the convention explicitly because an unlabeled pair of numbers can be misread.
The angle is arctan(rise ÷ run). It is not obtained by changing the percent sign to a degree symbol. A 45° slope has equal rise and run and therefore a 100% grade, illustrating how different the scales are.
Calculate a required level difference
Once a design supplies the target percentage, rearrange the relationship: vertical change = horizontal run × percentage ÷ 100. That computes the level difference corresponding to the specified geometry. It does not decide a suitable drainage or accessibility slope.
Record which end is higher and the reference elevation. The calculator reports a non-negative magnitude; the drawing and levels must establish direction. A correct absolute percentage applied in the wrong direction is still the wrong layout.
Check the whole path
A single end-to-end average can hide a low point, hump or change of gradient. For a path with multiple sections, calculate each segment using its own rise and run. Keep transition points and their elevations visible in the measurement record.
For paving, ramps, drainage and roofs, use the applicable project specification and responsible designer's details. Surface finish, crossfall, thresholds, outlets and local conditions are not assessed by a basic rise/run conversion. The tool is useful for interpreting dimensions after those requirements are defined.
Do it in order
- Measure horizontal run and the vertical difference between stated points.
- Convert them to matching units and calculate rise/run.
- Express the result as percent, a labelled vertical:horizontal ratio or an angle.
Planning boundary
These conversions do not choose a compliant ramp gradient or establish adequate drainage.
Sources checked
Is a 2% slope the same as 2 degrees?
No. Two percent means a ratio of 0.02, whose angle is about 1.146°. Convert through rise/run.
Can I use the sloping tape length as the run?
The formula requires horizontal run. Resolve the actual geometry instead of substituting slope length.